Does the coolant level go down? If not, since you say the oil looks clean, you might just have a build-up of condensation in the engine. Or maybe a little rain water got into it somehow? Down the exhaust maybe? I would recommend loosening the oil drain plug after the tractor sits overnight. hold a clean container under it and let a few drips come out of the plug. If there is any water or antifreeze in the oil, it should settle out and be in the bottom of the pan. If no antifreeze, you should be ok. Warm up the engine good and hot and then drain the oil. Refill with new oil and filter and see if steam improves.